1. Field of the Invention
The present invention relates to button type electric cells and, more particularly, to cells in which the chemical transformations undergone by the electrochemically active materials cause a change in their overall volume between the charged state and the discharged state. The term, electrochemically active materials, is used herein to mean both positive and negative active materials together with the electrolyte, provided that at least one of the components of the electrolyte takes part in the charge and discharge reactions.
2. Description of the Prior Art
The majority of button type cells consist of a first metal cup containing the positive active mass and acting as the collector of the positive electrode, a second metal cup containing the negative active mass and acting as the collector of the negative electrode, an electrolyte impregnated separator between the positive and negative active materials and an insulating seal between the two cups, one of which is fitted into the other. The seal simultaneously seals the assembly and electrically insulates the cup from each other.
In such cells, if the volume of the active materials changes between the charged state and the discharged state, it can follow according to the circumstances either that the button swells or, on the contrary, that a void is created inside the button. In order to avoid the first case, the cell may be provided with a casing whose size corresponds to the state where the active materials take up their maximum volume, but in all cases, when passing from the maximum volume state to the minimum volume state, the void which is generated inside gives rise to some disadvantages.
In the case of primary cells, the problem occurs particularly with those cells which have a soluble negative electrode and in which the reaction products of the negative electrode during discharge migrate to the positive compartment while the negative compartment is progressively emptied, beginning with the region next to the separator. If the migration of the discharge products to the positive compartment does not cause corresponding swelling of the positive active material, breaking of the ionic contact between the two electrodes can result, since the intervening gap has increased. This broken contact thus cause the discharge to stop prematurely.